Paperboard food containers are typically made by way of pressing a planar paperboard container blank in a matched metal heated pressware die set, as noted in one or more of U.S. Pat. No. 4,606,496 entitled “Rigid Paperboard Container” of R. P. Marx et al; U.S. Pat. No. 4,609,140 entitled “Rigid Paperboard Container and Method and Apparatus for Producing Same” of G. J. Van Handel et al; U.S. Pat. No. 4,721,499 entitled “Method of Producing a Rigid Paperboard Container” of R. P. Marx et al; U.S. Pat. No. 4,721,500 entitled “Method of Forming a Rigid Paper-Board Container” of G. J. Van Handel et al; and U.S. Pat. No. 5,203,491 entitled “Bake-In Press-Formed Container” of R. P. Marx et al. Equipment and methods for making paperboard containers are also disclosed in U.S. Pat. No. 4,781,566 entitled “Apparatus and Related Method for Aligning Irregular Blanks Relative to a Die Half” of A. F. Rossi et al; U.S. Pat. No. 4,832,676 entitled “Method and Apparatus for Forming Paperboard Containers” of A. D. Johns et al; and U.S. Pat. No. 5,249,946 entitled “Plate Forming Die Set” of R. P. Marx et al. The forming section may typically include a plurality of reciprocating upper die halves opposing, in facing relationship, a plurality of lower die halves. The upper die halves are mounted for reciprocating movement in a direction that is oblique or inclined with respect to the vertical plane. The paperboard blanks, after cutting, are gravity fed to the inclined lower die halves in the forming section. The construction of the die halves and the equipment on which they are mounted may be substantially conventional; for example, as utilized on presses manufactured by the Peerless Manufacturing Company. For paperboard plates stock of conventional thicknesses i.e., in the range of from about 0.010 to about 0.040 inches, it is preferred that the spacing between the upper die surface and the lower die surface decline continuously from the nominal paperboard thickness at the center to a lower value at the rim.
The paperboard which is formed into the blanks is conventionally produced by a wet laid paper making process and is typically available in the form of a continuous web on a roll. The paperboard stock is preferred to have a basis weight in the range of from about 100 pounds to about 400 pounds per 3000 square foot ream and a thickness or caliper in the range of from about 0.010 to about 0.040 inches as noted above. Lower basis weights and caliper paperboard is preferred for ease of forming and realizing savings in feedstock costs. Paperboard stock utilized for forming paper plates is typically formed from bleached pulp furnish, and is usually impregnated with starch and double clay coated on one side as is further discussed herein.
In a typical forming operation, the web of paperboard stock is fed continuously from a roll through a cutting die to form circular blanks which are then fed into position between the upper and lower die halves. The die halves are heated to aid in the forming process. It has been found that best results are obtained if the upper die half and lower die half—particularly the surfaces thereof—are generally maintained at a temperature in the range of from about 250° F. to about 400° F. These die temperatures have been found to facilitate the plastic deformation of paperboard in the rim areas if the paperboard has the preferred moisture levels. At these preferred die temperatures, the amount of heat applied to the blank is sufficient to liberate the moisture within the blank and thereby facilitate the deformation of the fibers without overheating the blank and causing blisters from liberation of steam or scorching the blank material. It is apparent that the amount of heat applied to the paperboard will vary with the amount of time that the dies dwell in a position pressing the paperboard together. The preferred die temperatures are based on the usual dwell times encountered for normal production speeds of 40 to 60 pressings a minute, and commensurately higher or lower temperatures in the dies would generally be required for higher or lower production speeds, respectively.
Paperboard for disposable pressware typically includes a coating. Illustrative in this regard are U.S. Pat. No. 5,776,619 to Shanton and U.S. Pat. No. 5,603,996 to Overcash et al. The '619 patent discloses plate stock provided with a base coat which includes a styrene-acrylic polymer as well as a clay filler as a base coat as well as a top coat including another styrene acrylic polymer and another clay filler. The use of fillers is common in the art as may be seen in the '996 patent to Overcash et al. In the '996 patent a polyvinyl alcohol polymer is used together with an acrylic emulsion as well as a clay to form a barrier coating for a paperboard oven container. See Column 12, lines 50 and following. Indeed, various coatings for paper form the subject matter of many patents including the following: U.S. Pat. No. 5,981,011 to Overcash et al.; U.S. Pat. No. 5,334,449 to Bergmann et al.; U.S. Pat. No. 5,169,715 to Maubert et al.; U.S. Pat. No. 5,972,167 to Hayasaka et al.; U.S. Pat. No. 5,932,651 to Liles et al.; U.S. Pat. No. 5,869,567 to Fujita et al.; U.S. Pat. No. 5,852,166 to Gruber et al.; U.S. Pat. No. 5,830,548 to Andersen et al.; U.S. Pat. No. 5,795,928 to Janssen et al.; U.S. Pat. No. 5,770,303 to Weinert et al.; U.S. Pat. No. 4,997,682 to Coco; U.S. Pat. No. 4,609,704 to Hausman et al.; U.S. Pat. No. 4,567,099 to Van Gilder et al.; and U.S. Pat. No. 3,963,843 to Hitchmough et al.
Various methods of applying aqueous polymer coatings and smoothing them are known in the art. See U.S. Pat. No. 2,911,320 to Phillips, U.S. Pat. No. 4,078,924 to Keddie et al.; U.S. Pat. No. 4,238,533 to Pujol et al.; U.S. Pat. No. 4,503,096 to Specht; U.S. Pat. No. 4,898,752 to Cavagna et al.; U.S. Pat. No. 5,033,373 to Brendel et al.; U.S. Pat. No. 5,049,420 to Simons; U.S. Pat. No. 5,340,611 to Kustermann et al; U.S. Pat. No. 5,609,686 to Jerry et al, and U.S. Pat. No. 4,948,635 to Iwasaki.
Most typically, die sets for forming paperboard containers include a male or punch die half and a female die half. Typically, the punch half is reciprocally mounted with respect to its opposing die half and both die halves are segmented. One or more portions of the die halves may be spring-biased if so desired, and the particular geometry of the die will depend upon the product desired. In this regard, there is shown in U.S. Pat. No. 4,832,676 to Johns et al. an apparatus for forming a compartmented paperboard plate. The dies illustrated in the '676 patent includes spring-biased segments as well as pressure rings on the punch half. The particular apparatus further includes articulated, full area knockouts.
As will be appreciated by one of skill in the art, the knock-outs are important for separating the finished product from the die halves, particularly during high speed operation. The mechanical features can be further augmented pneumatically as is disclosed in U.S. Pat. No. 4,755,128 to Alexander et al. Other patents of interest include:                U.S. Pat. No. 4,435,143 to Dempsey;        U.S. Pat. No. 5,041,071 to Reasinger et al. and        U.S. Pat. No. 4,778,439 to Alexander.        
As will be appreciated from the foregoing patents, mechanical full area punch knock-outs, located in the bottom area of the punch half, commonly are used to aid in the removal of the formed pressware products from the punch by actuating a short distance from the punch contour. The full area punch knock-outs typically separate the product from the punch contour successfully but can create a vacuum between themselves and the formed product resulting in slow and inconsistent release. In the worst cases, the formed product is retained in the die set and it is formed into the following product (double pressed). The double pressed product typically will release from the die set since it has more weight and can more easily break the vacuum. The double press products are scrapped by manufacturing or inadvertently packaged and sold to the consumer. Inconsistent product release from the punch can result in lower forming press productivity, increased product manufacturing cost and reduced useful package count to the customer.
Stated another way, the formed product can remain on or experience slow release from the one piece, full area punch knock-outs due to a vacuum created between its smooth surface contacting the product's smooth coated surface. The suction force created by the vacuum and knock-out surface area (force=pressure (vacuum)×area) may exceed the weight of a paperboard product. The product thus cannot release until air enters between the product and the knock-out or an air eject blow-off system slides the product sideways along the knock-out surface. Either way can result in slow and inconsistent product release.
Mechanical articulated punch knock-outs provide utility by more positively pushing the blank into the die set bottom to provide for full height product formation. This is especially useful in deeper products, such as bowls where heavy paperboard gathering (pleating) can prevent full height product formation. Articulated knock-outs can provide an even better seal/vacuum between the product and the punch knock-out, since they can wrap around into sidewall areas resulting in inconsistent product release and requiring other mechanical means (ie. split punch flange forming) and/or air assist to remove the products.
Removing the product with air can also have unexpected and inconsistent results. Air flowing across light weight pressware products can result in a creation of a “Bernoulli Effect” low pressure lift analogous to that experienced by an airplane wing. The air flowing through the small area between the product and punch can be at high velocity and generate low pressure based on Bernoulli's Principle of conservation of energy. This low pressure can create an upward force that can hold the product against the punch contour and knock-out. Air timing and pressure is critical to remove the products. More air pressure and flow may not provide better release since higher air flow velocities and lift forces can be generated. Just the right amount of air at the correct timing is required to remove the product from the punch, thus resulting in inconsistent results.
The formed pressware products tend to remain on the punch of the die set due to vacuum created between the punch contour, punch knock-out and coated paperboard products particularly. Sudden movement of the punch upwardly after product pressing can also result in the product remaining on the punch due to the vacuum. Improperly cured or tacky surface paperboard coatings can also result in the product sticking to/remaining on the punch half. The various mechanical knock-outs may not be able to remove products with improperly cured/tacky coatings. Corrective action in the coating operation or alternative materials are typically required to improve product removal from the punch half when such coatings are encountered.